Plant nutrition
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The nutrients
All gardeners know that plants require
regular supplies of nutrients yet plant nutrition is something that many choose
to ignore, much to the detriment of their gardens. Establishing the proper
nutrient balance not only ensures that your plants will grow well it also helps
to make them more resistant to climatic extremes, pests and diseases.
Plant growth requires certain elements
(nutrients) be present in the soil. The main, or macro, nutrients are nitrogen
(N), phosphorus (P), potassium (K), sulphur (S), calcium (C) and magnesium (Mg)
In addition there are other elements that are essential but only required in
minute quantities These most important of these trace elements, or
micro-nutrients, are iron (Fe), boron (B), molybdenum (Mo), manganese (Mn),
zinc (Zn), copper (Cu), chlorine (Cl) and iodine (I). All these elements are
distinct from the essential carbohydrates, hormones, proteins, etc. that plants
manufacture themselves.
Nitrogen, phosphorus and potassium are
regarded as the “big three”. You may have seen the letters NPK on fertiliser packets, this is a reference to the percentage of these
elements in the fertiliser blend. A fertiliser with an NPK of 20.10.15 has 20%
nitrogen, 10% phosphorus and 15% potassium. You will also find listed the types
of compounds in which the nutrient is available, i.e. nitrate or nitric
nitrogen, potassium in nitrate, phosphate or chloride forms. This information
is important in determining how quickly and for how long the nutrients will be
available.
It is important to understand that these
nutrients do not “feed” the plant in the same sense that we feed ourselves.
Some of the nutrients are the raw ingredients with which the plant, through
photosynthesis, is able to convert light into energy and chemical compounds
that result in growth. Other nutrients are important in the construction of
cells or in cell division.
When it’s said that a plant is showing
signs of, say magnesium deficiency, that doesn’t mean the plant is able to
utilise magnesium directly. It’s rather that it needs magnesium to be able to
produce chlorophyll, the green plant pigment that is vital for photosynthesis.
That’s why a plant lacking magnesium appears yellow; it’s lacking chlorophyll.
A similar effect can be seen on many deciduous plants in autumn when the supply
of elements such as magnesium is reduced in reaction to the natural seasonal
cycle thereby causing dormancy through leaf fall. Often deciduous plants
growing near light sources, such as street lights, will retain some of their
foliage over winter because the added light offsets the nutrient reduction
enough to enable photosynthesis to continue.
Some plants have adapted to grow in soils
that are deficient in particular elements, for example most Proteaceae prefer soils low in
phosphate. Other plants have evolved methods that enable them to make up for
deficiencies, for example all plants can use atmospheric nitrogen but
leguminous plants can store it in special nodules on their roots thus making up
for nitrogen deficient soils.
Despite this ability to adapt all plants
need all of the essential elements. Some, such as iodine and
zinc are required in such minute quantities that you will probably never need
to add them to your soil. Others such as cobalt and molybdenum are also
only required in minute quantities but may be absent in some soils leading to
stunted growth.
The following is a list of the essential
nutrients and their effects.
Major elements
Nitrogen
Vital for many aspects
of plant health, such as the production of chlorophyll, amino acids and
hormones. Plants lacking nitrogen have a general
yellow appearance and their growth becomes stunted. New foliage becomes
progressively smaller until growth stops.
Phosphorus
The exact effects of phosphorus are not
clear but it appears to activate enzymes and hormones and may act as a growth
regulator. A lack of phosphorus leads to poor and distorted growth.
Potassium
Needed for the proper
division of cells and in the production of chlorophyll, carbohydrates and
proteins. A lack of potassium results in stunted
and plants with distorted foliage. A characteristic feature is yellowing around
the edge of the leaves.
Calcium
Vital for maintaining
healthy cell walls and removing waste products.
Although calcium is often supplied as lime (calcium chloride) even acid soil
plants need calcium. For these plants calcium sulphate or calcium nitrate are
preferable. but light doses of dolomite lime will
proved calcium (and magnesium) with raising the pH excessively.
Magnesium
Involved in the
production of chlorophyll and as an activator for enzyme changes. Lack of
magnesium causes stunted pale foliage and chlorosis that eventually develops
into brown patches on the foliage. This is probably best described as an autumn
leaf appearance.
Sulphur
Essential for proper root development and
ensures that foliage colour is maintained. Much of a plant’s sulphur are obtained from the air in the form of sulphur dioxide or,
in the case of coastal plants, in sulphate form from ocean spray.
Minor elements
Boron
The precise function of boron is unclear
but it appears to be closely linked to the movement of sugars through plant
tissues. A lack of boron leads to the breakdown of internal cells typified by
discolouration and rotten or hard patches, particularly in potatoes, parsnips
and cauliflowers. In trees and shrubs persistent tip die-back is the most
obvious symptom of boron deficiency. Pines grown as timber trees may also
develop pockets of resin in born deficient soils but this is not fully proven.
Only minute amounts of boron are required for healthy growth—one application
(in the form of borax or boric acid) every fifteen years or so is usually
sufficient.
Copper
Copper’s ability to alternate between its
cupric and cuprous states is believed to be a vital part of the process of
photosynthesis. A lack of copper causes stunted and distorted growth with
blotched and twisted leaves. If left untreated it leads to die-back of the
young growth.
Chlorine
This element stimulates the action of
photosynthesis. Without it photosynthesis slows to an eventual halt resulting
first in wilted foliage that rapidly develops “autumnal” colours then dies.
It’s rare to see chlorine deficiency but should it occur it requires rapid
corrective action as any affected plants are, in effect, switched off.
Iron
Iron is necessary for the most important
plant functions; the manufacture of chlorophyll, photosynthesis and
respiration. Iron deficiency rapidly leads to chlorosis. Foliage gradually
yellows but the leaf vein usually remain green. This
starts with the older leaves but gradually spreads to the whole plant.
Manganese
Essential for the
production of chlorophyll and maintaining cell structure. Plants lacking manganese become chlorotic and develop scabby
patches of dead cells. The easiest way to tell the difference between manganese
chlorosis and most other chlorotic conditions is that manganese chlorosis
appears on the new growth first. This is because manganese unlike iron and some
other elements is fixed, it cannot be transported around the plant, so if there is no manganese the new growth doesn’t get any
and becomes chlorotic. With iron chlorosis the iron is sent to the growing tip
so that area remains healthy while the old growth becomes chlorotic.
Molybdenum
This element is important in the fixation
and transportation of nitrogen. The symptoms of molybdenum deficiency are quite
mild; a general slowing down and gradual lightening of the foliage colour.
Iodine
Only needed in
extremely minute amounts. It’s highly unlikely
that any soils would have so little iodine as to be deficient. It may not
actually be an essential element but it is a component of certain important
amino acids.
Zinc
Only required in very small quantities
but important in the production of chlorophyll and essential growth controlling
auxins (hormones). A lack of zinc will lead to an inability to utilise other
nutrients and will eventually shut down growth entirely. As with many of the
elements that contribute to chlorophyll the first sign of zinc deficiency is
yellowing foliage.
It is important to realise that an excess
of these nutrients may be just as damaging as a deficiency. Some trace elements
are, in fact, used as weed killers.
In addition to these elements some plants
may also need cobalt (Co), silicon (Si) or sodium (Na). The most important of
these in New Zealand horticulture has been cobalt. Battling cobalt deficiency has been
an important feature in central North Island
agriculture. Farmland in this area of volcanic soils was highly deficient in
cobalt. Crops grew well enough but vital fungi and algae were lacking this led
to grazing stock became severely diseased. The discovery of the cause of this
problem combined with the advent of aerial topdressing has turned this area
into prime farming country. However, cobalt deficiency is rarely a problem in
home gardens.
Sources of nutrients
There are two main groups of plant
fertilisers; organic and chemical, or to put it another way, natural and
artificial. Both forms are available in dry and liquid forms. Dry fertilisers
are almost always worked into the soil or used as a soil dressing. Liquid
fertilisers are often used applied to the soil too but many are intended to be
applied to the foliage. These are known as foliar fertilisers or foliar feeds.
Organic fertilisers
Naturally occurring organic fertilisers
tend to be relatively mild unless applied in large quantities. Many have the
benefit of adding humus as well as nutrients but remember some are quite low in
nutrient value and others offer only a limited range. You may need to add
chemical fertilisers to ensure a good supply of nutrients. The most common
organic fertilisers are:
Compost
Either home made or bought this is
probably the best all round product for adding nutrients and conditioning the
soil. Any garden waste can be used to produce compost but the best materials
are fallen leaves or conifer needles, vegetable waste and finely chopped plant
trimmings.
Animal manures
Animal manures should be well weathered
before use. Not only is fresh manure very unpleasant to handle it is also quite
likely to burn fine surface roots. Animal manures frequently contain weed seeds
but cow manure, which is the result of a very thorough digestive process, is
usually almost weed-free.
Green manures
Some crops, particularly the nitrogen
fixing legumes, are grown for the express purpose of digging in when mature.
This adds to humus to the soil but much of the nitrogen fixed by the crop is
lost is the process of decomposition.
Processed products
The above mentioned organic products are
frequently processed into more easily handled forms. Such pre packaged
fertilisers are often the best buy for those with small gardens as they require
only limited storage area and little or no preparation.
Chemical fertilisers
Although organic products are better at
adding humus chemical fertilisers are more suited to providing a balanced
supply of nutrients and for correcting specific deficiencies. There appears to
be no firm evidence to suggest that plants react any differently to, or are
adversely affected by, direct chemical applications as opposed to the more
subtle organic products. Chemical fertilisers come in two types.
General blends
General chemical fertilisers supply a
balanced blend of nutrients. They are an ideal as a dressing before planting
and as a booster in general cultivation. Some mixes are intended for specific
plant groups, such as acid soil plants or orchids.
Nutrient specific fertilisers
These fertilisers supply one element or a
specially selected group of element, such as the trace elements, and are
primarily intended to correct specific deficiencies. An example would be the
use of iron sulphate or iron chelates to correct iron chlorosis.
An alternative use is to provide a
nutrient in a form that is acceptable to a particular type of plant. For
example calcium is normally supplied in the form of lime, which makes the soil
more alkaline. Many ornamentals cannot tolerate alkaline soils so calcium must
be supplied in another form. Dolomite lime is a mild alternative to hydrated lime
but genuinely calcifuge plants will not even tolerate dolomite lime. If such
plants are lacking calcium it is normally applied as calcium ammonium nitrate.
The correct use of nutrient specific
fertilisers requires a good understanding of plant nutrition but gives almost
unlimited scope for making fine adjustments.
The ideal for most gardens is probably a
blend of organic fertilisers for bulk and humus with chemical fertilisers to
make up for any deficiencies or for plants with specific requirements.
Efficient composting
Theory of composting
If there’s one thing that gardeners
consistently underestimate it’s the benefits of humus. Nothing improves the
soil more than natural compost. It’s both a fertiliser and a soil conditioner.
Visit any woodland area and look at the marvellous loose black soil that forms
where the fallen leaves are allowed to build up and decay.
Composting can be as simple or technical
as you care to make it, from simple rotted grass clipping to a precisely
balanced blend, but it need never be a major drain on your time. It’s a natural
process that can for the most part be left to nature. Your job is to get the
materials together and assemble them in a way that will allow the decomposition
to be as effective as possible.
The trick is to build a heap that will
reach a good heat. At its centre a compost heap may reach 60° - 70° C. or more,
this is hot enough to kill most harmful bacteria and weed seeds and also speeds
the rate of decay. By turning the heap so that fresh material is moved to the
centre rather than just rotting on the surface it’s possible to produce an almost sterile compost.
Pure vegetable compost has some nutrient
value but better results will be obtained if additional bulk manures can be
added. This is especially true when the compost is made from on site materials
year after year. Obviously nutrients are lost through harvesting so an
injection of added fertiliser material will be necessary. Cow manure is the
perfect supplement as it is reasonably weed free and quick rotting but other
animal manures or chemical fertilisers are satisfactory.
Composting problems usually arise because
the heap has been poorly made or badly maintained. This leads to it failing to
reach a high enough temperature through being too wet, too dry or inadequately
aerated. Judging the correct moisture content is not difficult once the heap is
up and running but it’s important to avoid any problems before this stage by
building the heap properly in the first place.
Compost bin designs
At a pinch you can get by with a heap of
compost material on bare earth but it’s far better to
contain it in some way. There’s no set design for compost bins but I prefer the
modular crate design for the following reasons:
* Aeration. Oxygen is
needed for the various soil organisms to decompose the compost quickly and
completely. In the crate design the gap between the slats means that good
aeration is built in. A lack of adequate aeration is a problem with the
tumbler, or drum, design. Tumblers tend to ferment the compost rather than
properly decompose it.
* Easy access. The
crate design allows the front to be removed completely making it easy to turn
the heap and to remove the compost. Also there’s no difficulty in adding water
or fertilisers and it's easy to monitor the temperature.
* Modular design. This
allows two or more crates to be operating at once so that there is always
somewhere to put fresh material.
* Long life and low maintenance. The use of treated timber (H4) ensures that the crates will last and require virtually no maintenance (use galvanised nails).
* Simple construction. Anyone moderately competent with a saw and hammer should have little trouble assembling a simple crate.
If you have very limited space then a
tumbler or drum may be more suitable otherwise the crate design or some
modification of it offers the simplest solution as it allows for compact tidy
storage and maximum efficiency.
Making the compost
You’ll get reasonable results even if you
just throw a pile of material in the bin and leave it there to rot but your
compost will be far better if you put a little more thought into it. The most
efficient method of compost making is layering. Layering is preferable because
it gives you greater control over the moisture level and consistency of the
compost.
The idea is to build up a “lasagne” of
compost with each layer a complete mini-heap in itself. You can stop after any
layer and not have to worry about finding enough material to fill the bin but it’s better if you can entirely fill one bin then move onto
the next.
Start with a fairly dry, coarse material
on the bottom. This gives good drainage and acts as a “nest” to support the
rest of the layer. Next add your wetter materials; vegetable peelings, bulky
foliage and rotting fruit. If you tend to build up masses of old newspapers its quite acceptable to put a layer of shredded paper
between the dry base and the wet materials.
Each layer should be composed of a dry
lower section, a moist centre with a thin crust of soil on top. If you have
slightly drier material, such as grass clippings put it on top of the wet
material then finally top off with a thin layer of soil. Each completed layer
will be about 40 cm - 60 cm deep and be composed of three or four types of
material.
While building the heap you can add
various fertilisers to speed up decomposition and enrich the final product. A
high nitrogen fertiliser will speed the decomposition of the bottom dry layer
while any animal manure or chemical fertilisers should be added with the wetter
material. A dressing of lime on top of the soil layer will ensure that the
compost is not too acid. Put the lime on top as it rapidly leaches though the
heap.
When you have finished building the heap
cover it with a weatherproof cover. This will stop it getting wet and help to
prevent flies getting into the cooler top parts of the heap. It will also
prevent weeds growing on top of the heap.
Do not add weeds or diseased material to
your first compost heap, see how hot it gets first. If
your heap doesn’t build up and hold a good heat (over 55° C.) it won’t kill
weed seeds or fungal spores. If your heap does heat up well then virtually any
compostable material can be included.
Monitoring the heap
You will need to turn the after two to
four weeks depending on the time of year but you should check its progress
after a week or so. A soil thermometer is a useful aid, make a small hole and
place the thermometer in the centre of the heap. It should read at least 55° C.
If it’s much lower then you may need to adjust your mix next
time, it may be too dry. For now add a little water or liquid
fertiliser. If it’s hotter than 55° C. that’s fine.
Examine the base of the heap, if there’s
liquid oozing out it’s too wet, next time add more dry matter or less wet. A
little seepage is nothing to worry about but small rivulets of brown ooze show
there is too much liquid present.
Turning the heap will, of course, break
up the structure of the layers but by then it shouldn’t matter; the composting
process will be well under way. It’s more important at that stage to make sure
all the material spends time in the hot centre of the heap to make the compost
as consistent and sterile as possible.
Efficient use of household waste
Much of the time you will not have enough
material to fill an entire compost bin in one swoop. Garden waste may come in
large quantities but household waste is usually just a pile of vegetable
peelings or old fruit; not enough to fill a compost
bin. What can you do with small quantities?
There are two methods that I would
recommend. The first is to dig a trench in a part of your vegetable garden that
you are not currently using. Put the old vegetable peelings and the like in the
trench and when full cover dig in the material leaving another trench behind
the first. Then fill the next trench, dig in and make another, progressing across
the garden. This way you dig your garden and recycle your waste.
The second method is to have extra
compost bins that can act as holding bins until enough material has been built
up to make proper compost. The household waste will tend to be fairly wet and
may be somewhat unpleasant to work with in a partially rotten state but at
least you don’t have unsightly trenches of waste matter in the garden.
Making garden mulches
Its not just green
matter that can be composted. Sawdust, bark, dry
pine needles, spent potting mix and, as mentioned above, newspaper can all be
composted to make a less nutrient rich drier compost
that makes a good mulch.
You can, of course, add these things to
your regular compost but if you want to make a consistent
mulch it’s often better to compost them separately. This is quite
straightforward but may take considerably longer than garden waste and need
larger heaps to generate the same amount of heat.
Wood products, such as bark and fresh
sawdust, will require considerable amounts of extra nitrogen to make them rot
quickly. Add about 500g of urea or 1kg of sulphate of ammonia for cubic metre
of material. Extra fertilisers can also be added if you wish. Your bins will
need to be able to hold at least 2 cubic metres, preferably more, in order to build up a good heat. Allow four to six months to compost, turning once a month.
Using this method it is also possible to
compost hardwood prunings but you will need some way of chopping them into
small pieces. Most hardware shops stock garden shredders but unless you have
regular large amounts to shred it may be a false economy to spend a large
amount on a shredder. For the cost of a shredder you could buy at least ½ a
truckload of ready-made mulch.
Composting is not simply a matter of
doing your bit for recycling. It’s also the best, and ultimately the cheapest,
way of building up your soil’s humus content and thereby improving its nutrient
content, aeration, structure, moisture retention and drainage. No garden should
be without a compost heap and no serious gardener should be without compost.
Copyright Geoff Bryant
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